Bentonite/cement grouts and slurries are used in the civil engineering industry in various ground engineering processes. The specifications usually require a minimum compressive strength or shear strength at 28 days age. The interaction between the materials namely bentonite, cement and water is complex; both the bentonite content and the cement content of the grout affect the development of shear strength. An investigation of the development of shear strength has been made for a range of grouts with bentonite contents from 5% to 20% by weight of water and water/cement ratios from 4 to 12. The shear strength was measured at various ages up to 336 days age using a Pilcon hand held shear vane tester, for many grout mixes the shear strength was still increasing at 336 days age. Two mixes were placed in vertical plastic pipes in order to investigate how shear strength may vary with depth in a ground borehole. A mathematical model has been developed to predict the shear strength of any grout mix at any age within the range studied, the model comprises a simple equation with two coefficients A and B which can be obtained from contour plots depending on the mix proportions. The model has been tested using four mixes different from those use to derive the model; statistically there is no significant difference between the actual and predicted results and the model can be used with reasonable confidence.